Support CertManager for the Webhook cert generation

[tenzen-y]

/kind feature

Currently, the training-operator generates certs for the webhook using the internal cert generation mechanism for all installation ways.

However, we know that some administrators prefer to use the cert-manager to manage all certifications for security and so on.
So, I would suggest supporting the cert generation mechanism by the cert-manager.

xref: #2035

[tenzen-y]

/kind feature

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[tenzen-y]

/remove-lifecycle stale

[andreyvelich]

/good-first-issue

[google-oss-prow]

@andreyvelich:
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